Heating pads and electric blankets commonly use heating elements powered by AC line voltage, where the temperature of the heating elements is controlled, and the safety continuously monitored, to protect against overheating. Typically, power to the heating element of such devices is switched on by a solid state switch, e.g., a triac. The integrity of the triac is a key factor in the safety of the product. Should a triac fail in a shorted condition, continuous heating and, in turn, overheating can result, with the risk not only of exposure to the user causing him/her to possibly suffer burns, but also the chance of a fire. It is, therefore, important to detect a shorted power switch condition and disconnect the power before allowing an unsafe condition to develop.
Using multiple circuits in a heating element of a heating pad or electric blanket, it has been found, provides better detection of overheating due to a larger portion of the heating wire being affected by a bunch condition. In this manner, overheating can often be recognized sooner by the temperature control circuitry.
U.S. Pat. No. 5,420,397 to Weiss, for instance, discloses a safety circuit for a positive temperature coefficient (PTC) heater wire that detects a break in the wire, and quickly turns the power off before an arc can cause the wire, which is highly flammable, to catch fire. The circuit uses a triac to switch the power on and off in time proportion relative to the heat setting. In one embodiment, two triacs are used in series to mitigate the effect of one of the triacs failing by becoming shorted, inasmuch as the other of the two triacs would disrupt the power. In another embodiment, a second triac is used in a crowbar circuit to open the power fuse in the event that power to the PTC wire is detected during the “off” state of the power control triac.
Heating pads and electric blankets usually have higher wattage than is needed to stabilize at the desired temperature. The extra power is typically provided to quickly bring the surface of the heating pad or electric blanket up to the desired temperature. Such is termed a “preheat mode” which drives the heater wire to a higher temperature for a short period of time. After cessation of the preheat mode, a controller measures the temperature and maintains the wire at a target temperature according to a setting selected by the user. In this case, the power required to maintain the desired temperature may be as little as 20% of the total available power. A solid state switch, commonly a triac, can fail by becoming shorted in either a full wave or half wave condition. Even a failure in the half wave condition could continuously provide 50% of the power and eventually result in overheating of the heating element. While attempts have been made to detect a triac short in the positive half cycle only, such attempts, it has been found, often leave the circuit vulnerable to a situation where the triac may be shorted in the negative half cycle, and a runaway temperature could result.
Appliances other than heating pads and electric blankets have heating elements powered from an AC line and use triacs to switch the power on and off to control temperature by connecting the heating element to AC power when the temperature is below a preset value, and disconnecting the heating element from the AC power when the required temperature is reached. Other types of AC operated appliances use electronic AC switches to operatively connect and disconnect power to the load. Failure resulting from a shorted electronic power switch (e.g., a triac) in such appliances can also lead to an unsafe and uncontrollable temperature rise in the heating element, or other unsafe conditions.
In microcontroller (MCU) based circuits, an MCU is used to measure temperature of the heater wire and provide a control signal for the triac. These MCU circuits are quite often powered by a non-isolated low voltage power supply connected to the power line, providing just a single polarity DC voltage, e.g., +5V. Having a single polarity power supply provides obstacles for direct detection of the opposite polarity or bipolar signals.
In situations where a heating pad (or other appliances having a heating element powered from an AC line) is used, it has been found advantageous to use two circuits, where one circuit is powered by the positive half cycle of the AC power line, typically 120 VAC, and the other circuit is powered by the negative half cycle of the AC power line, as described in U.S. Patent Application Publication No. US 2013/0134149A1 to Weiss. Heating elements typically used have positive temperature coefficient characteristics, for example, when nickel is used, and the temperature is determined by the measured resistance of both circuits. A first circuit resistance is measured during the positive half cycle of the AC power line, and a second circuit resistance is measured during the negative half cycle. The requirement that the resistance be measured in the negative half cycle for the second circuit has, in turn, led to a requirement that conduction of the triac be determined both as to when power should be applied and when power should not be applied. Conductance of the triac when the power should not be applied is indicative of a triac short. Triacs can be shorted for either the positive or negative half cycles of the AC line, or even for both cycles. For single circuit and multiple circuit heating pads, the omission to detect AC switch failure for either half cycle may lead to the power generated during that cycle being applied to the heating element, and cause overheating.
The same principles apply generally to other types of the electronic AC switches, e.g., MOSFET based AC switches, BJT based AC switches, thyristor based AC switches, triac equivalents, etc.
With an appropriate circuit arrangement, the MCU can detect failures and shut down the circuit under certain abnormal conditions, such as overheating, a wire break, or when the triac fails short.
With reference to FIG. 1, which illustrates generally a conventional circuit, MCU U1 triggers triac Q1 to energize heating element or heater HT1 in response to a reading of a temperature sensor (not shown). MCU U1 periodically stops the trigger pulses to evaluate the condition of triac Q1. A Triac Detector circuit 10 comprising a transistor Q2, reverse voltage protection diode D3, current limiting resistor R2 and a load resistor R1, detects the presence of voltage on an MT2 terminal of triac Q1, and outputs a low level to the MCU when voltage is present, and a high level when there is no voltage (triac conducting). The MCU compares the response of the triac detector circuit to the current status of a control signal (trigger) of triac Q1. If the control signal is present, and the triac is expected to conduct current, the MCU expects a high level from the detector. When there is no control signal, the MCU expects a low level from the detector, during the positive half cycle of the power line voltage. Triac short failure is detected when the MCU receives the high level while the triac is not triggered. When such failure is detected, the MCU triggers the crowbar circuit, comprising a triac Qc and a current limiting resistor Rlim, which blows Fuse F1 and disconnects the heater HT1 from the power line.
While useful, the circuit of FIG. 1, is not responsive to a negative voltage at MT2 terminal of triac Q1. The triac, has both negative and positive switching facilities (for positive and negative half cycles of the power line) that can fail independently. However, the circuit of FIG. 1 does not protect against the short failure of the negative cycle control part of triac Q1. Thus, there is a need to provide detection of the triac failure for both the positive and negative half cycles of the power line.